Structural and electronic properties of doped NiO

How the electronic structure of correlated oxides evolves with electron and hole doping is a fundamental question of importance for applications. We have studied structural and electronic properties of high-quality MBE-grown doped NiO thin films using a variety of experimental and theoretical methods. We find that both K- and In-doped films at concentrations below 10% exhibit good X-ray diffraction spectra. However, extended X-ray absorption fine structure (XAFS) spectra indicate broadening of the K-O bond length distribution, and an increased bond length, much larger than obtained using theoretical methods. Ab-initio molecular dynamics suggests the XAFS feature is due to K-O_v-K defect structures in the thin film. Spectroscopic ellipsometry shows a smooth but asymmetric evolution of a decreasing optical gap for hole (K) and electron (In) doping, consistent with the behavior of a charge transfer insulator. Analysis shows that the decrease of the optical gap is due to the emergence of an absorption peak below (in energy) the optical gap of un-doped NiO. Calculations using Dynamical Mean Free Theory are in good agreement with the measurements and show the emergence of occupied states just above the valence band edge, and with an asymmetry between hole and electron doping.